Here's a radical "fix" that will work if you don't mind losing a file or program... I had accidentally selected S4PE (package editor) as the default program for .ts4script files. Didn't make them not work, but, it wouldn't DO anything to them either. There's no way to UNSELECT a program (to get back to the "blank icon" beside those files, I mean).
I didn't know what else to switch them to... so today, I deleted S4PE. Icons, back to normal. (I use Sims4Studio anyway.) It isn't the BEST solution, probably, especially if you're talking about some true default program, but for an add-on like s4pe that I didn't desperately want to keep, it worked fine.
(Outside the box idea.)
